1. Field of the Invention
The present invention relates to production methods for producing multilayer ceramic substrates, and in particular relates to a method for producing a multilayer ceramic substrate having a cavity with multi-steps for mounting and housing an electronic component.
2. Description of the Related Art
In recent years, there has been a growing need for further reduction in size and weight, further enhanced multi-functions, higher reliability and the like, of electronic devices, and thus a need for improving technologies for mounting electronic components on a substrate exists. Most typically, an effective way of improving the mounting technologies is to achieve a high wiring density of the substrate.
To address high wiring density of such a substrate, a multilayer ceramic substrate which is fabricated by stacking, pressing and then firing a plurality of green ceramic sheets, each having a printed conductive film and the like thereon, has been developed.
In order to achieve a reduction in the size and thickness of the multilayer ceramic substrate, it is effective to form a cavity for mounting an electronic component in the multilayer ceramic substrate. When such a cavity is formed so as to have multi-steps, that is, when the cavity is formed so as to have at least a first cavity segment and a second cavity segment communicating with the bottom face of the first cavity segment, the bottom face of the first cavity segment can be used, for example, as a region for forming a conductive pad for wire-bonding an electronic component while the electronic component is housed in the second cavity segment.
However, as a result of firing during the manufacture of the substrate, a multilayer ceramic substrate having such a cavity with multi-steps often encounters such problems in that undesirable deformation or fracture develops at the periphery of the cavity, and a crack occurs at the end of the bottom face of the cavity, though this is not as noticeable when the multilayer ceramic substrate has a cavity with a single step.
The above problems are assumed to arise as a result of residual stress at a specific portion of a green sheet laminate. That is to say, when the green sheet laminate to be a multilayer ceramic substrate is pressed before sintering, it is difficult to put uniform pressure on the entire green sheet laminate because a relatively uneven cavity with multi-steps exists in the laminate. This results in deterioration of the flatness of the stepped bottom face of the cavity or in undesirable deformation of the cavity.
To solve the above problems, Japanese Unexamined Patent Application Publication No. 9-39160 discloses a pressing step which is performed such that a green sheet laminate having a cavity with multi-steps is vacuum-packed while being sandwiched by a pair of rubber sheets and is pressed isotropically in a stationary fluid.
Japanese Unexamined Patent Application Publication No. 9-181449 discloses a pressing step which is performed such that a green sheet laminate having a cavity with multi-steps is pressed by an elastic member having a projection with multi-steps, the projection having the same shape as the cavity with multi-steps.
Japanese Unexamined Patent Application Publication No. 6-224559 discloses a method for fabricating a green sheet laminate that is pressed entirely and that has a cavity with multi-steps. The method comprises the steps of placing a rigid plate, which has a through-hole equal to or slightly smaller than the opening of the cavity, on the green sheet laminate and pressing the green sheet laminate from an elastic member via the rigid plate by exerting a pressure thereon at every step of the cavity.
However, even when the method stated in Japanese Unexamined Patent Application Publication No. 9-39160 is adopted, one of the rubber sheets intrudes into the cavity in the pressing step, resulting in a stress on the stepped bottom face of the cavity and an undesirable deformation of the bottom face. Accordingly, this sometimes makes it difficult for the green sheet laminate to maintain the flatness of the bottom face of the cavity. In addition, intrusion of the rubber sheet may cause the cavity to widen its radial dimension and, as a result, a deterioration in its dimensional accuracy can occur.
The method stated in Japanese Unexamined Patent Application Publication No. 9-181449 requires preparing elastic members corresponding to shapes of the cavities to be formed in the multilayer ceramic substrate. Furthermore, the pressing step requires aligning the cavity having multi-steps formed in the green sheet laminate with the projection having multi-steps formed in the elastic member; however, this alignment is not so easy, and accordingly hampers attempts to increase the efficiency of the step. With this view, it is expected that the method according to the Japanese Unexamined Patent Application Publication No. 9-181449 will entail an increased production cost of the multilayer ceramic substrate.
Japanese Unexamined Patent Application Publication No. 6-224559 requires pressing the green sheet laminate at every step of the cavity, thus giving rise to a problem of relatively low production efficiency, although the flatness of the stepped bottom face of the cavity is maintained and the radial dimension of the cavity does not widen.
Accordingly, it is an object of the present invention to provide a method for producing a multilayer ceramic substrate having a cavity with multi-steps therein.
The present invention, in brief, has a feature such that an appropriate block is inserted in a portion of a cavity with multi-steps so as to allow the cavity to have an apparent single step and thus to perform a pressing step.
More particularly, a method for producing a multilayer ceramic substrate according to the present invention comprises the steps of (a) fabricating a green sheet laminate by laminating a plurality of green ceramic sheets, a stack of predetermined green ceramic sheets having through-holes therein so as to provide a cavity in the green sheet laminate, the cavity having an opening at one end face of the green sheet laminate in a lamination direction thereof and comprising at least a first cavity segment and a second cavity segment communicating with the bottom face of the first cavity segment, (b) preparing a block having substantially the same three-dimensional shape as that of the cavity with the first cavity segment excluded and having a height equal to or larger than the depth of the cavity with the first cavity segment excluded, (c) inserting the block into the cavity with the first cavity segment excluded, (d) pressing the green sheet laminate in the lamination direction thereof, and (e) firing the green sheet laminate.
The foregoing block can be made from any material as long as the material has characteristics to maintain the formation of cavity to a certain extent in the step of pressing the green sheet laminate. The block may comprise an inorganic material which does not sinter at the sintering temperature of the ceramic material contained in the green ceramic sheet, or a hardening resin that burns out at the sintering temperature of the ceramic material contained in the green ceramic sheet.
The pressing step according to the present invention is preferably performed as follows. It may comprise the sub-steps of preparing an elastic member and preparing a rigid plate having a through-hole that is equal to or slightly smaller than the opening of the cavity. While the through-hole of the rigid plate is aligned with the opening of the cavity, the pressing step is performed such that the rigid plate is placed on the green sheet laminate and a pressure is exerted on the green sheet laminate from the elastic member via the rigid plate.
Hydrostatic pressing is preferably applied in the pressing step. In this case, the green sheet laminate, the rigid plate and the elastic member are put into a vacuum-packed state in a packing container.
The cavity formed in the green sheet laminate according to the present invention may have three steps or more. For example, the cavity may further comprise a third cavity segment communicating with the bottom face of the second cavity segment.
The present invention is directed to the production mention for producing a multilayer ceramic substrate by using a so-called non-shrinkage process. In this case, the production method according to the present invention may further comprise the step of preparing a shrinkage-inhibiting inorganic material which does not sinter at the sintering temperature of the ceramic material contained in the green ceramic sheet, wherein the fabricating step includes providing shrinkage-inhibiting layers comprising the shrinkage-inhibiting inorganic material so as to cover both end faces of the green sheet laminate in the lamination direction, while one of the shrinkage-inhibiting layers has a through-hole therein so as to expose the opening of the cavity. Then, the firing step is performed under the condition that only the ceramic material contained in the green ceramic sheet sinters.
In the preferred embodiments as described above, the block may comprise the shrinkage-inhibiting inorganic material.